Currently Navy ships employ a separate antenna for each function resulting in a proliferation of a large number of antennas on the ships to meet the numerous functional requirements. Recently, there is a significant interest to develop multi-function arrays using a single wideband antenna aperture, e.g. as described in G. Tavik, J. Alter, S. Brockett, M. Campbell, J. DeGraff, J. B. Evins, M. Kragalott, et al, “Advanced Multifunction Radio Concept (AMRFC) Program Final Report”, NRL Memo Report, NRL/FR/5303—07-10,144 (June 2007). However, the number of radiating elements needed to avoid grating lobes, at the highest frequency of this wideband antenna aperture, becomes prohibitively large resulting in a complex and costly multi-function array. There is some effort to reduce the number of elements using frequency scaled arrays (see, e.g., B. Cantrell, J. Rao, G. Tavik, M. Dorsey and V. Krichevsky, “Wideband array antenna concept”, IEEE Aerospace and Electronic Systems Magazine, vol. 21, no. 1, pp. 9-12 (2006) (“Cantrell et al.”); R. Kindt, M. Kragalott, M. Parent and G. Tavik, “Preliminary investigations of a low-cost ultrawideband array concept”, IEEE Transactions on Antennas and Propagation, vol. 57, no. 12, pp. 3791-3799 (2009) (“Kindt et al. 1”); and R. W. Kindt, M. Kragalott, M. G. Parent, and G. C. Tavik, “Wavelength-Scaled Ultra Wideband Antenna Array”, PCT/US09/64154 (November 2009) (“Kindt et al. 2”)), but such approaches are limited, e.g. the latter being limited to symmetric and/or square arrays. In one approach, the operating frequencies are chosen to be a factor of two apart, limiting the flexibility of the derived architectures.
Carrier class US Navy ships have the following satellite communication (SATCOM) link requirements. A link is needed to set up a direct path of communication between a shipboard antenna and a satellite. A carrier needs to have links for the following functions:                TV-links at both C and Ku-bands        Commercial links at C and Ku-bands        Navy links at X and K-band, and        Navy MetOc (Meteorological and Oceanographic) links at L and S-bands        
At least one link needs to be formed at each one of these frequencies. Table 1 lists the frequencies of interest as well as the antenna aperture size required to satisfy the directivity requirements. From Table 1 it can be seen that the C-band function needs the largest aperture size of 25.6 m2.
TABLE 1SPECIFICATIONS OF SATCOM DOWNLINKS FOR A CARRIERNotionalMaximumDownlinkNotionalApertureInter-ElementFrequencyDirectivitySizeSpacing (mm)System(GHz)(dB)(m2)dx × dyCommercial 3.7-4.2 (C)47.025.635.7 × 35.7 10.7-12.75 (Ku)49.05.211.8 × 11.8TV 4.08-4.127 (C)41.05.336.3 × 36.312.224 (Ku)43.01.012.3 × 12.3Navy 20.2-21.2 (K)52.02.97.1 × 7.1 7.25-7.75 (X)46.05.219.4 × 19.4MetOc1.684-1.71 (L)32.03.987.7 × 87.72.205-2.2535 (S)34.03.666.6 × 66.6
For a rectangular lattice, e.g. as described in M. I. Skolnik, ed. “Radar Handbook”, 2nd Ed., McGraw Hill, Boston Mich., pp. 7.17-7.25 (1990), the inter-element spacing for grating lobe free operation in both the x- and y-directions, can be calculated using Equation (1):
                              d          x                =                              d            y                    =                                    1              2                        ×                          c                              f                highest                                                                        (        1        )            In Equation (1), c is the speed of light (=3×108 m/s) and fhighest is the highest frequency in the bandwidth of operation. The variables dx and dy represent the maximum inter-element spacing in the x- and y-directions respectively. Table 1 also lists the maximum inter-element spacing allowed for each function to ensure that the antenna pattern is grating lobe free over the entire bandwidth of operation. For example, to operate over the C-Band (3.7-4.2 GHz) the inter-element spacing can be at most 35.7 mm. A smaller inter-element spacing will also satisfy a grating lobe free operation, but a lot more elements will be needed to satisfy the directivity specification requirement.
If it is desired that a single aperture is designed to handle all the frequencies, then the radiating element used in the aperture will need to work from the lowest frequency of 1.684 GHz to the highest frequency of 21.2 GHz. Using the formula in Equation (1), the maximum inter-element spacing in this case will depend on the highest frequency, which is 21.2 GHz and will be equal to dx=dy=7.1 mm. This element will need to operate over a bandwidth of
  12.6  ⁢      :    ⁢  1  ⁢            (              =                              21.2            1.684                    ⁢                      :                    ⁢          1                    )        .  If an element of dimensions 7.1×7.1 (mm2) were used to fill the largest array aperture of 25.6 m2 required to satisfy the directivity at C-band, then almost 510,000 elements will be needed. This large number of elements will make this multi-function array very complex and costly.
In a conventional architecture as illustrated in FIG. 1, an element is channelized for each link that needs to be formed. In this example, eight links are needed, thus the output of each element will need to feed eight separate beamformers or in other words, the output of each element will feed eight phase shifters, eight attenuators etc. This extremely large number of components needed to form this multi-beam architecture further illustrates the complexity and high cost of a conventional multi-functional array.
Carrier Architectures
In an attempt to reduce the number of elements, the invention adopts the approach of using frequency scaled radiating elements which has also been adopted and discussed by Cantrell et al. and Kindt et al. 1-2. However, the method discussed by Cantrell et al could not be used here because of the constraint that requires equal beamwidth for all frequencies and arrays, which is not the case for the functions considered here. Strictly speaking, the method discussed by Kindt et al. 1-2 is too stringent for the desired application because it is designed to have equal beamwidth for functions at different frequency bands. However, the procedure of frequency scaling as used by Kindt et al. 1-2 can be modified for the problem at hand.
From Table 1, it can be observed that the inter-element spacing needed at K-band (20.2-21.2 GHz) is approximately ½ the size of the inter-element spacing needed at Ku-band (10.7-12.75 GHz). Similarly the inter-element spacing needed at Ku-band is about ⅓ the inter-element spacing needed at C-Band (3.7-4.2 GHz). The inter-element spacings needed at the other frequency bands lie somewhere in between the above two values. This means that an array with inter-element spacing designed for Ku-band can provide grating lobe free operation at all frequencies below 12.75 GHz. In similar vein, an array designed with inter-element spacing at C-band will provide grating lobe free operation at all frequencies below 4.2 GHz. Now, following the method discussed in Kindt et al. 1-2, symmetry is maintained in the array aperture. To maintain this symmetry the array with the smallest inter-element spacing (for the highest frequency) is either positioned at the center or at one corner of the multi-function phased array aperture. In the example shown in FIG. 2, the array with the smallest inter-element spacing (also referred to as the core) is positioned in the bottom right corner of the Multi-Function array (in this case, the C-band array). Next, the array designed to have the next larger inter-element spacing forms a layer around the perimeter of the core. Finally the outer-most layer will have the largest inter-element spacing.
FIG. 2 shows the inter-element spacings used for different sections of the Multi-Function phased array aperture. The core will have elements with the smallest inter-element spacing (i.e. x×x) where from Table 1,
  x  =                    11.8        ⁢                                  ⁢        mm            2        =          5.9      ⁢                          ⁢      mm      followed by the second perimeter having inter-element spacing of 2x×2x. Finally the outer-most region will have inter-element spacing of 6x×6x. The value of 5.9 mm is chosen over the maximum allowed inter-element spacing of 7.1 mm for K-band because we want to keep whole number multiples between the inter-element spacings of the different regions as suggested by the designs in Kindt et al. 1-2. If the core has an inter-element spacing of 7.1 mm, then with a multiple of two, the inter-element spacing of the next outer layer will need to be 14.2 mm. This inter-element spacing will ensure no grating lobe formation for X-band and other lower frequency arrays. However, at Ku-band, this inter-element spacing is larger than the maximum allowed of 11.8 mm for grating lobe free operation and hence will result in the formation of grating lobes. By the same argument, using a whole number ratio of two between the inter-element spacing of the middle layer and the outer layer, the outer most layer will have an inter-element spacing of 2×2×7.1 mm=28.8 mm, which is smaller than the needed 35.7 mm. A smaller inter-element spacing will result in the need for more elements to satisfy the directivity requirement. To avoid this, an inter-element spacing of 11.8 mm of the middle layer is selected as the basis. This means, that now the inter-element spacing in the core will be half of 11.8 mm (i.e. 5.9 mm) while the inter-element spacing in the outer most layer will be three times 11.8 mm (i.e. 35.4 mm).
Since the core has the elements with the smallest inter-element spacing, reducing this spacing will result in a significant increase in the number of elements needed to satisfy the directivity requirement. To avoid this, fractional multipliers are applied between inter-element spacings of the different arrays. This will be discussed in more detail further below.
Note, that since the area required to satisfy the directivity for TV (Ku) function is smaller than the area of the K-band array, it is better to use only a portion of the K-band array. If the entire array were to be used, then more directivity than needed will be obtained, which is a bonus, but at the same time more phase shifters, attenuators and other components would also be needed. This will unnecessarily make the system more complex and costly. A similar reasoning can be used for the L and S-band arrays, which are smaller than the X/Ku/TV(C) band arrays.
By using the architecture where the inter-element spacings are frequency scaled, it is possible to reduce the number of elements significantly. Using frequency scaled architectures, as shown in FIG. 2; the total number of elements are reduced from 510,000 to only 116,110, which is almost a 77% decrease in the number of elements needed to form 8 beams. One of the difficulties in implementing this architecture is that the radiating elements in the core region need to have a bandwidth of 12.6, which is very difficult to achieve. Another issue is that the core of this architecture needs to be able to form eight links simultaneously. At present, there are no simple and cost effective beamforming techniques that are capable of forming eight simultaneous beams with very small element spacing (5.9 mm) needed for this design. The emergent simple and cost effective beamforming technology at present is only capable of providing a maximum of four simultaneous beams (see Kindt et al. 2). Still another point of concern is the fact that low frequency links such as the L and S-bands which are able to provide grating lobe free operation even at large inter-element spacings (87.7 mm and 66.6 mm respectively) are being forced to use much smaller elements and inter-element spacings. This significantly increases the number of elements needed at these frequencies and hence also increases the number of components needed, increasing the cost and complexity of the arrays. At the same time, there is a large area of the C-band array that has only one link on it while a small corner of the array is forced to provide eight links.